On Power Electronics Interface for Distributed Generation Applications and its Impact on System Reliability to Customers

Licentiatavhandling, 2005

Distributed generation (DG) is being employed as a means of achieving increased reliability for electrical power systems as regarded by consumers. As the most of DG technologies utilize renewable sources, the power electronic interface plays a vital role to match the characteristics of a DG unit with the grid requirements.
In this thesis, a voltage source converter (VSC) is considered as a front end for a DG unit that utilizes a variable speed wind turbine. The control of the voltage/current is considered the most important part when implementing the interface. In order to obtain a high bandwidth, a vector current controller (VCC) is developed for the VSC connected to the grid through a filter inductor. The VSC system is simulated and examined in case of voltage dips. The VCC has proven to work adequately in case of balanced dips. However, most faults are unbalanced resulting in unbalanced voltage dips on the VSC terminals. Hence, the VCC had to be modified to give a better performance in case of grid voltage imbalance.
The dual vector current controller (DVCC) has been implemented to enhance the performance in case of unbalanced voltage dips. Two methods are proposed and compared to calculate the current references used by the controller. They depend on how the oscillating power, which is the power at double the fundamental frequency that is produced due to unbalanced faults, is compensated. The system is examined for all possible voltage dips. Design equations are derived to calculate the maximum current that the VSC switches should hold in case of different dips. Moreover, a case study is presented at which a design criterion, based on the knowledge of wind statistics at a specific site, is introduced to give the DG the capability to ride-through faults.
The controller is then modified to control a VSC connected to the grid through an inductor-capacitor-inductor (LCL-) filter, which has the advantage of eliminating the higher harmonics in the grid current. The VSC connected to a weak grid through an LCL-filter is also considered in the thesis. The voltage regulation limits are calculated and the controller is tested in case of the load connection/disconnection and balanced voltage dips.